Various instruments or devices can be implanted in a patient's body and require through-the-skin connection between the implanted device and an external device or instrument. A percutaneous access device (PAD) can be employed to provide such connection. A PAD is employed on a long-term or permanent basis to provide external pneumatic and electrical connection for implanted devices such as a cardiac assist device of the type described in U.S. Pat. No. 4,630,597. Percutaneous access devices of this general type are disclosed in U.S. Pat. Nos. 4,634,422 and 5,242,415, and consist basically of a body having a circular flange implantable beneath the patient's skin and a projection projecting coaxially along one side of the flange through the patient's skin. Electrical and/or pneumatic leads from within the patient's body pass through a passage in the projection of the PAD to a point where the electrical and/or pneumatic leads are accessible from the exterior of the patient's body.
As set forth in greater detail in U.S. Pat. No. 4,634,422, in order to implant the PAD, at some stage during the implantation procedure, it is necessary to cut a circular opening through the patient's skin to provide a passage for the projection. The epidermis, the outer layer of skin, has a natural propensity to attempt to close this surgical opening. U.S. Pat. No. 4,634,422 describes in detail a technique for firmly bonding the underlying dermal layer of skin to the sidewall of the projection to prevent the downgrowth of epidermis around the implanted device.
The formation of a bond between the dermis and the projection of the PAD is a delicate and time consuming operation which, as might be expected, requires a precise and uniform initial "fit" between the edge of the circular opening cut through the dermis and the outer periphery of the projection of the PAD. The diameter of the opening through the dermis is, ideally, closely matched to the outer diameter of the projection by the selection of a trephine of appropriate size to cut the opening through the skin.
However, due to the resilience of the skin, conventional trephines do not cut an opening with truly cylindrical sidewalls. Instead, the trephine produces an opening of a frusto-conical configuration with the larger diameter end of the opening facing outwardly of the skin. The outer sidewall of a PAD projection or analogous device is typically cylindrical or tapered with the larger diameter next to the inside of the skin. Because of this, the "fit" between the projection of the PAD and the dermis is loose in an area where it has the greatest need to be tight. This greatly increases the possibility of delayed or inadequate bonding between the skin layer and the projection, as well as increasing the risk of development of infection.
In U.S. Pat. No. 5,139,508 to Kantrowitz et al., a surgical tool for forming a circular opening through the skin having a diameter which increases with depth from the exterior surface of the skin is proposed. The device disclosed in Kantrowitz '508 is directed to a trephine with a cutting blade which is inserted into a pocket surgically formed beneath the skin. A rod member is inserted through the skin and threaded into the trephine. The rod member is attached to a spring biased plunger within a tool housing. Upon release of the plunger, the spring drives the plunger so as to drive the cutting edge of the trephine outwardly through the skin. This device produces the desired reverse taper opening. however, it is also desirable to control the taper angle with a greater degree of precision than that routinely obtainable with the device described in Kantrowitz '508. Additionally, this device requires assembly during surgery thus prolonging the procedure. Also there is a risk of parts being lost in the body.
It would be desirable to provide a trephine device which can produce the desired reverse taper holes by cutting conveniently from the outside. It is further desired that the device require little or no assembly and/or disassembly during surgery. Furthermore, it is desirable that the device permit the taper angle to be customized from application to application such that an angle of essentially zero or a near perpendicular cut may also be obtainable. Finally, it is desirable that the trephine device produce minimal tissue trauma and assist in the preparation of a subcutaneous pocket suitable for receiving the desired implant device, wherein the aperture placement is specifically oriented relative to the pocket thereby permitting easy and rapid installation of the implant after the reverse taper hole has been produced.